1. Field of the Invention
The present invention relates in general to a heat exchanger for an air conditioner.
2. Description of the Prior Art
With reference to FIG. 1, there is shown the construction of a conventional heat exchanger for an air conditioner. As shown in the above drawing, the conventional heat exchanger includes a plurality of regularly spaced flat fins 100. The fins 100 are vertically arranged such that they parallel each other. A plurality of heat transfer pipes 102 are angularly fitted into the fins 100 such that the pipes 102 are perpendicular to the fins 100. The air currents flow in the space defined between the fins 100 in the direction of the arrow A in FIG. 1 and exchanges heat with the fluid flowing in the heat transfer pipes 102.
A thermal fluid flowing around each flat fin 100 is characterized in that the thickness of the thermal boundary layer 103 on both heat transfer surfaces of the fin 100 is gradually thickened in proportion to square root of the distance from the air current inlet end of the fin 100 as shown in FIG. 2. In this regard, the heat transfer rate of the fin 100 is remarkably reduced in proportion to the distance from the air current inlet end. Therefore, the above heat exchanger has a lower heat transfer efficiency.
When lower velocity air currents flow in the direction of the arrow of FIG. 3, the thermal fluid flowing about each heat transfer pipe 102 is characterized in that the air currents separate from the outer surface of the pipe 102 at portions spaced apart from the stagnation point of the pipe 102 at angles of 70.degree.-80.degree. . Therefore, a cavitation zone 104 is formed in the back of the pipe 102 as shown in the region of FIG. 3 designated by cross hatching. In the cavitation zone 104, the heat transfer rate of the pipe 102 is remarkably reduced so that the heat transfer efficiency of the above heat exchanger becomes worse.
In order to overcome the above problems, Japanese U.M. Laid-open Publication No. Sho. 55-110995 proposes an improved heat exchanger for air conditioners. As shown in FIGS. 4 and 5, the Japanese heat exchanger includes a plurality of heat transfer pipes 112 which are fitted into the regularly spaced flat fins 110 (only one fin depicted) such that the pipes 112 are perpendicular to the fins 110. The above heat exchanger also includes a plurality of slit type grilles which are formed beside the pipes 112 on each fin 110. Each slit type grille is formed by vertically slitting a given portion of the fin 110 several times and alternately bending the remaining strips in opposite directions, thereby forming a plurality bent strips 115a, 115b, 115c, 115d, 115e and 115f in the fin 110.
That is, three strips 115a, 115c and 115e are bent to one side of the fin 110 such that the strips 115a, 115b and 115c are regularly spaced apart from each other. However, the other three strips 115b, 115d and 115f placed between the above strips 115a, 115c and 115e are bent to the other side of the fin 110.
The above-described heat exchanger having the plurality of slit type grilles on each flat fin 110 causes the heat exchanging fluid flow to become turbulent due to the above grilles, thereby reducing the thickness of the thermal boundary layers formed on the fins 110. The pressure of thin thermal boundary layers somewhat improves the heat transfer efficiency in comparison with the conventional heat exchanger having the flat fins 110 with no slit type grilles. When the partial heat transfer capacities of the heat exchanger are measured, the upstream strips 115a and 115b form the thin thermal boundary layers, thus improving the heat transfer efficiency. However, as the downstream strips 115c to 115f are included in the thermal boundary layers formed by the upstream strips 115a and 115b, the downstream strips 115c to 115f can not improve the heat transfer efficiency. In addition, the cavitation zone is still formed in the back of each heat transfer pipe 112. Furthermore, the air currents flowing in the space defined between the flat fins 110 are laminar and do not become mixed together. Therefore, the above Japanese heat exchanger is not expected to provide the improved heat transfer efficiency as could occur if the air currents are mixed together.